The present invention relates to paper machines and methods of operating the same.
In particular, the present invention relates to the drying section of a paper machine and to a method for operating the same.
The present invention relates especially to the initial part of the drying section which includes at least one group of drying cylinders, a fabric loop formed from a suitable drying fabric means or felt being guided around these cylinders which are arranged in two main rows in such a way that the cylinders of one row are situated in alignment with spaces between the cylinders of the other row. The fabric means travels in zig-zag fashion around the above group of cylinders in a manner situating one row of cylinders within the loop of the fabric means and the other row of cylinders outside of this loop. The arrangement is such that the web is at all times supported by the fabric while travelling therewith around the above rows of drying cylinders so that there is no open draw for the web within said group of cylinders.
Thus, the present invention relates to a drying section which has two main rows of drying cylinders arranged horizontally with one row situated at an elevation higher than the other and with the cylinders of one row situated in alignment with the spaces between the cylinders of the other row, a drying fabric means in the form of a suitable drying wire or felt being guided in zig-zag fashion around the cylinders to situate one cylinder row within the loop of the fabric means and the other outside of this loop, at least those cylinders which are situated outside of the loop being smooth-surfaced cylinders which are heated in a suitable manner and which are directly contacted by the web over a given sector of the latter cylinders.
As is shown, the drying section of the paper machine usually includes two main rows of smooth-surfaced drying cylinders which are arranged horizontally and located with one row above the other. The paper web is conducted so as to travel in zig-zag fashion around the drying cylinders so that the web alternately travels around the cylinders of the upper and lower rows while directly contacting all of these cylinders. The web is pressed against the surfaces of the cylinders over a given sector thereof by utilizing so-called drying felts or drying fabrics of which at least one is provided for the cylinders of the upper row and at least one is provided for the cylinders of the lower row. The fabric means referred to in this application is intended to apply to all fabrics, whether wires or felts, which are appropriate for this purpose, regardless of their material and manner of manufacture.
With such an arrangement the web is not supported at all times. Instead there is an open draw while the web travels from the cylinder of one row to the cylinder of another row. Thus, the web is in such an event exposed in the space between the cylinder rows to those air currents which are created by the rotating cylinders. As a consequence, there occurs in many cases a powerful fluttering of the web, to such an extent that the web may break and cause a shut down in the production. Attempts have been made to eliminate this drawback by arranging at the initial part of the drying section, said free or open draws to be shorter than is the optimum length of said draws as regards the drying process. In this connection reference may be made to Finnish Pat. No. 45,585.
In order to improve the reliable operation of a paper machine, it has been also proposed to provide an arrangement, which has already been tried, wherein the web is conducted at the initial part of the drying section, for example within the first cylinder group, in such a way as to be supported by a fabric means at all times. In this case this particular cylinder group has only a single fabric means which is common to the cylinders of both the upper row and the lower row. Thus with such an arrangement the fabric means presses the web against the surfaces of the cylinders of one row, for example, the upper row, while at the lower row the fabric means is situated between the web and the cylinders of this lower row.
With an arrangement of this latter type it is possible to achieve a remarkable improvement in the reliability of the operation of the paper machine. However, this improvement is achieved at the expense of the desired drying effect, inasmuch as at the lower row of cylinders, in the above example, the web is not directly in contact with the drying cylinders but is separated therefrom by the fabric means. Thus the improvement of reliability in operation achieved by such an arrangement is based on the fact that the fluttering of the web is prevented at the space between the upper and lower rows. However, it has been found that in connection with paper machines which operate at high speeds the web tends to detach itself from the fabric means while travelling around these cylinders, as a result of centrifugal force or also due to the weight of the web at the lower cylinder row. This centrifugal force increases with increasing speed and with a decreasing diameter of the drying cylinders. The detachment of the web from the fabric means is partly due to the fact that in the throat between the fabric means and the surface of the cylinder there is generated a relatively high pressure which tends to detach the web from the fabric means.
The present invention relates to a method and apparatus according to which the web is transported in closed conduction, without any open draw, at least at the initial part of the drying section.
With respect to the state of the prior art which may pertain to the present invention, reference may be made to U.S. Pat. Nos. 3,503,139 and 3,874,997.
U.S. Pat. Nos. 3,503,139 shows a construction according to which a drying cylinder group, as is conventional, has two rows of cylinders situated one above the other and having smooth surfaces while being provided only with a single lower drying wire. The paper web passes through this cylinder group in such a way that the drying wire presses the web directly against the surfaces of the lower cylinders whereas at the upper row the web is sepated from the surfaces of these upper drying cylinders by the drying fabric. In order to attempt to compensate for the impairment in drying efficiency resulting from the above arrangement, at the cylinders of the upper row, these cylinders of the upper row are operated at a temperature higher than those at the lower row. The result is that the steam system of the paper machine is more complex than usual. It hss also been attempted to improve the drying effect by providing for the cylinders of the upper row a housing which may contain a means for providing for the blowing of hot air against the surface of the web.
Thus, the purpose of the above patent is simply to improve and enhance, by way of heat technology, the drying process while at the same time attempting to achieve a paper of higher quality. The disclosure of this patent is not directed toward improving the reliability of the operation of the paper machine.
With respect to U.S. Pat. No. 3,874,997, there is a construction according to which the cylinder group has also a pair of horizontal rows of drying cylinders through which the web travels while being supported by one drying wire only. The cylinders in the lower row have a smooth surface and insofar as these cylinders are concerned the drying of the web is carried out by contact drying. The cylinders of the upper row, however, are permeable to air, and with respect to these upper cylinders the drying is in part carried out by flow of air through the cylinder shells and in part by impingement of air on the web, for which purpose there is provided in this patent a structure enclosing the upper row of cylinders in a hood which blows hot air. Thus this patent also relates primarily to an improvement in the drying process, with respect to the heat technology thereof.